1. Field of the Invention
The present invention is directed to an apparatus for measuring weak biomagnetic fields which are physiologically generated by an examination subject, the biomagnetic fields appearing topically and chronologically in the body of a patient, and in particular to an apparatus which permits such fields to be measured while the patient is in an exercise condition and in a resting condition.
2. Description of the Prior Art
It is known to measure the chronological course of electrical pulses emanating from the beating heart using electrodes applied to specific locations on the body surface, and to record this signal in an electrocardiogram. Frequently such measurements are undertaken with the patient being at rest and exercising, so as to obtain an "at rest" ECG and an exercise ECG. The latter measurement is taken immediately after the patient has undergone defined physical exercise, usually using an ergometer so that the exercise level can be recorded. Because the electrodes affixed to the body are connected to the ECG measuring instrument via flexible lines, these measurements are independent of the position and movements of the body during each measurement.
Electrocardiograms produced in this manner are diagnostically valid with respect to the amplitude and frequency of the pulses which are generated by the heart. Such electrocardiograms are not suitable, however, for localizing specific electrical events occurring in the heart muscle, and thus cannot be used to provide conclusive information as to the spatial position of electrical events and their chronological course.
It is also known to employ magnetic methods for measuring and analyzing local bioelectrical currents in the biological tissue complexes, particularly the brain and the heart, and to perform such and measurements so as to produce magneto-encephalograms (MEG) and magneto-cardiograms (MCG).
A known apparatus which is suitable for this purpose is described in an article entitled "Biomagnetismus" by Hohnsbein in the periodical Bild der Wissenschaft, No. 8, 1986, pages 76-83. This known apparatus is capable of measuring extremely weak biomagnetic signals, for example, the magnetic fields which arise in living tissue complexes due to directed current flows, having a field strength on the order of magnitude of 10.sup.-12 T and below. These signals can be measured only by special sensor arrangements when the patient and measuring equipment are carefully shielded from external magnetic fields. The sensor arrangement consists of a plurality of measuring devices known as gradiometers so that an exact localization of the current source in the tissue can be identified. These gradiometers are coupled to a corresponding number of SQUIDs (superconducting quantum interference devices). Both the gradiometer and the associated SQUID must be accommodated in a cryostatic temperature regulator, in which a temperature prevails at which the SQUID and the gradiometer are superconducting. The error with which the current source can be localized has a defined relationship to the number of sensors, i.e., to the number of measuring points, as explained in the article "MCG Inverse Solution, Influence of Coil Size, Gride Size, Number of Coils, and SNR," Abraham-Fuchs, IEEE Transactions on Biomedical Engineering, Vol. 35, No. 8, August 1988, pages 573-575, particularly section C of that article in combination with FIG. 5 of the article. In order to obtain a localization error which is sufficiently low for clinical use, the magnetic signals must be simultaneously measured with at least 10 to 12 channels dependent on the signal-to-noise ratio.
Among other reasons, because of their low temperatures, magnetic sensors, differing from ECG electrodes, can not be affixed to the body of the patient and connected to a central unit via flexible lines. By contrast, these magnetic sensors must be arranged at a specified distance from the examination subject. This means that the body of the examination subject cannot change spatially relative to the magnetic sensors during a measuring event. This means that the body of the patient must be situated in a fixed position relative to the magnetic sensors, including the time when measurements are undertaken after exertion on the part of the patient. It is necessary to undertake an exercise measurement immediately after the exertion by the patient insofar as possible. Consequently, it is not possible to lead the patient to a support table after the ergometer exertion, and to affix the patient at that location in order to subsequently register the MCG. The time which passes would be too long to permit a diagnostically relevant exercise measurement to be taken.
In other medical areas, it is known to provide an exercise mechanism at the foot end of a patient's bed, to permit the bed-ridden patient to avoid the formation of embolisms after an operation. Such exercise equipment, however, does not include an ergometer which permits a diagnostically relevant measurement of an exercise ECG or an exercise MCG to be taken.